The MDM2-related protein MDM4 contributes to p53 inactivation during embryonic development (1). In contrast to MDM2, however, MDM4 is only expressed at low/undetectable levels in most adult tissues (2) and is largely dispensable for adult tissue homeostasis (3-6). Therefore, whereas MDM2 functions in both proliferating and terminally differentiated cells, MDM4 assists MDM2 in suppressing p53 only in highly proliferating cells such as during embryonic development or in the proliferative compartment of the intestinal epithelium (7). Consistently, MDM4 is expressed in the highly proliferating mouse Embryonic Stem (mES) cells and its expression decreases upon retinoic acid-induced differentiation (8).
MDM4 expression is often increased in cancer cells as one mechanism to inhibit p53-mediated tumor suppression. MDM4 mRNA expression is elevated in a substantial fractions of human tumors such as stomach and small intestine cancers (43%), glioblastomas (8%), colorectal cancers (20%) or breast cancers (20%) (9-12), but also, e.g., in lung cancer, osteosarcoma and melanoma (see particularly FIG. 4a of ref. 9; Table 2 of ref 10; Table 1 of ref. 11). The mechanism(s) that promote MDM4 expression in human tumors are not fully understood although of great interest as potential therapy targets. One such mechanism is gene amplification, occurring, for instance, in a small fraction of breast cancers (9). It was recently demonstrated that MDM4 protein, but not mRNA, levels are elevated in ˜65% of cutaneous melanomas (13). This observation indicates that post-transcriptional mechanisms can also contribute to increased MDM4 expression in a subset of cancers (and emphasizes the need to develop a diagnostic assay that measures MDM4 at the protein level, which may be more difficult to implement in routine diagnostic labs); importantly, it also raises the possibility that thus far, the frequency of MDM4-expressing cancers has been underestimated, as most studies have focused on reporting MDM4 gene copy number variations and total mRNA levels.
This recent study established a causative link between MDM4 overexpression and melanoma formation in vivo and, importantly, underlined the addiction of melanoma cells to high levels of MDM4. MDM4 silencing decreased melanoma growth and this was, at least partly, a consequence of increased p53-dependent apoptosis. Consistently, targeting the physical interaction between MDM4-p53 by using SAH-p53-8, a small cell-penetrating stapled alpha-helical peptide, was sufficient to induce p53-dependent apoptosis in melanoma cells (13).
Although targeted therapy with BRAF-selective inhibitors such as vemurafenib has recently yielded impressive anti-tumor responses in melanoma patients carrying BRAFV600E mutations (14, 15), drug resistance is typically acquired within 12 months (16). Relapses can be postponed, but usually not avoided, when vemurafenib is combined with a selective MEK1/MEK2-inhibitor such as cobimetinib (17). Overcoming resistance to targeted therapies is likely to require targeting of multiple oncogenic mechanisms. Importantly, SAH-p53-8 sensitized melanoma cells to conventional chemotherapeutics and to inhibition of BRAFV600E by vemurafenib and inhibited growth of BRAFV600E-mutant melanoma cells that acquired resistance to BRAFV600E-inhibitors (13). These data indicate that targeting the MDM4-p53 interaction represents a unique therapeutic opportunity to reactivate suppressed p53 function in the context of anti-melanoma combination therapy. Since MDM4 is expressed in many other cancers as well (9-12), this strategy would be applicable in other tumors than melanoma as well.
For MDM2 inhibitors in development, side effects such as nausea, vomiting, fatigue, anorexia, insomnia, electrolyte imbalance, and mild renal/liver function impairment have been reported (Tabernero et al., 2009). MDM2 inhibition in normal tissue in mice leads to increased levels of apoptosis in the gut (Mendrysa et al., 2003), which leads to worries about therapeutic safety of these inhibitors. Indeed, a concern of therapies that aim to restore wild-type p53 activity is that these might lead to widespread apoptosis in normal tissues.
MDM4 might, therefore, be a safer and more promising anti-cancer therapeutic target. While it is not expressed in most normal adult tissues, many cancer cells, e.g., 65% of melanoma, up-regulate MDM4 to dampen p53 tumor suppressor function.
Unfortunately, small molecules that selectively and efficiently disrupt the MDM4-p53 complexes have so far not been identified/introduced into the clinic. Moreover, there is an increasing body of evidence that MDM4 possesses p53-independent oncogenic functions (2, 18-21). Consistently, in addition to induce p53-dependent apoptosis MDM4 silencing in melanoma cells also caused cell cycle arrest, that could not be rescued upon concomitant inactivation of p53 (13). Inhibition of melanoma growth upon MDM4 KD was more prominent than that seen upon inhibition of the MDM4-p53 interaction and could also be observed in some mutant p53 melanoma cells. These data point to p53-independent mechanisms of MDM4 oncogenicity in melanoma, in addition to its well-known ability to suppress p53.
Therefore, as an alternative to pharmacological inhibition of the MDM4-p53 protein interaction, which has proven to be very challenging, it was reasoned that targeting MDM4 abundance may not only be easier to achieve pharmacologically (and, therefore, easier to introduce into the clinic), but may have broader and more robust antitumor effects as this would inhibit both p53-dependent and independent oncogenic functions of MDM4.
It would be advantageous to identify compounds that can inhibit cancer growth, lack the toxicity associated with MDM2 inhibitors, and address both the p53-dependent and independent oncogenic functions of MDM4.